Far-infrared spectroscopy device

ABSTRACT

This far-infrared spectroscopy device is provided with: a variable wavelength far-infrared light source that generates first far-infrared light; an illuminating optical system that irradiates a sample with the first far-infrared light; a detecting nonlinear optical crystal that converts second far-infrared light into near-infrared light using pump light, said second far-infrared light having been transmitted from the sample; and a far-infrared image-forming optical system that forms an image of the sample in the detecting nonlinear optical crystal. The irradiation position of the first far-infrared light on the sample does not depend on the wavelength of the first far-infrared light.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/743,151, filed Jan. 9, 2018, which is a 371 of InternationalApplication No. PCT/JP2015/070794, filed Jul. 22, 2015, the disclosuresof all of which are expressly incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a far-infrared spectroscopy device.

BACKGROUND ART

Electromagnetic waves in a far-infrared region ranging from 25 μm to 4mm in a wavelength are also called terahertz waves and have both radiowave permeability and light straightness. Since the absorption spectrumin this region has peaks inherent in many substances, theelectromagnetic waves in the far-infrared region are expected to beeffective for identification of substances. However, in the related art,there was no compact and easy-to-use light source that emits light inthis region, and a detector was also necessary to cool with liquidhelium or the like, which made the light source difficult to handle.Therefore, in the related art, electromagnetic waves in a far-infraredregion was used only for limited research applications.

In the 1990s, a light source and detector using a femtosecond laserwhich are small and do not require cooling were put to practical use.Currently, general-purpose spectroscopic measuring devices based ontime-domain spectroscopy are also on the market, and research anddevelopment for various fields such as security, bio-sensing,medicine/pharmaceutical, industry, agriculture, and the like areunderway. In such industrial applications, quantitative analysis ofcomponents is required.

CITATION LIST Patent Literature

PTL 1: JP-A-2003-302666

SUMMARY OF INVENTION Technical Problem

Quantitative analysis of components is one key in industrialapplications. There is an attempt to use quantitative analysis withtime-domain spectroscopy, but this method has problems that it isdifficult to measure 1 to 3 THz waves which are effective for detectionof hydrogen bonding or molecular network, to measure through a shieldsuch as paper and packaging materials, and to measure powder with strongscattering. On the other hand, the method using a variable frequencycoherent light source is easy to obtain high output in the region of 1to 3 THz, and is effective for analysis through a shield and analysis ofpowder.

However, with the above method, when the frequency is changed, thedirection in which far-infrared light comes out changes, thus there is aproblem that an irradiation position to a sample changes, and theaccuracy of quantitative analysis is lowered.

Therefore, the present invention provides a far-infrared spectroscopydevice capable of reducing the shift of an irradiation position offar-infrared light by frequency change.

Solution to Problem

For example, in order to solve the above problem, the configurationdescribed in the claims is adopted. Although the present applicationincludes a plurality of means for solving the above-mentioned problem,as an example thereof, there is provided a far-infrared spectroscopydevice that includes a variable wavelength far-infrared light sourcethat generates first far-infrared light, an illumination optical systemthat emits the first far-infrared light onto a sample, a nonlinearoptical crystal for detection that converts second far-infrared lightfrom the sample into near-infrared light by using pump light, and afar-infrared light imaging optical system that images the sample on thenonlinear optical crystal for detection, in which an irradiationposition of the first far-infrared light on the sample does not dependon a wavelength of the first far-infrared light.

Advantageous Effects of Invention

According to the present invention, shift of an irradiation position offar-infrared light due to frequency change may be reduced. Furtherfeatures relating to the present invention will become apparent from thedescription of this specification and the accompanying drawings. Inaddition, the problems, configurations, and effects other than thosedescribed above will be clarified by the description of the followingexamples.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a side view of a configuration example of a spectroscopydevice using light in a far-infrared region in a first example of thepresent invention.

FIG. 1B is a top view of a configuration example of a spectroscopydevice using light in the far-infrared region in the first example ofthe present invention.

FIG. 1C is a plan view showing an irradiation region of a sample in thefirst example of the present invention.

FIG. 2 is a view showing how far-infrared light propagates in the firstexample of the present invention.

FIG. 3A is a view for explaining a method of adjusting an incident anglewhen far-infrared light transmitted through the sample is incident on anonlinear optical crystal for detection in the first example of thepresent invention.

FIG. 3B is a view for explaining a method of adjusting the incidentangle when far-infrared light transmitted through the sample is incidenton the nonlinear optical crystal for detection in the first example ofthe present invention.

FIG. 4A is a view showing how far-infrared light is generated in thefirst example of the present invention.

FIG. 4B is a view showing respective wave vectors of far-infrared light,pump light, and seed light.

FIG. 5 is a view showing a configuration example of an incident angleadjusting optical system in the first example of the present invention.

FIG. 6 is a view showing a configuration example of the spectroscopydevice using light in a far-infrared region in a second example of thepresent invention.

FIG. 7A is a view for explaining an irradiation position of thefar-infrared light on a sample surface in the second example of thepresent invention.

FIG. 7B is a view for explaining the irradiation position of thefar-infrared light on the sample surface and the correction methodthereof in the second example of the present invention.

FIG. 7C is a view for explaining the irradiation position of thefar-infrared light on the sample surface and the correction methodthereof in the second example of the present invention.

FIG. 8 is a view showing a configuration example of a light source forpump light of a variable wavelength far-infrared light source in thesecond example of the present invention.

FIG. 9A is a side view of a configuration example of the spectroscopydevice using light in the far-infrared region in a third example of thepresent invention.

FIG. 9B is a top view of a configuration example of a spectroscopydevice using light in the far-infrared region in the third example ofthe present invention.

FIG. 9C is a plan view showing an irradiation region of a sample in thethird example of the present invention.

FIG. 10A is a side view of a configuration example of a spectroscopydevice using light in a far-infrared region in a fourth example of thepresent invention.

FIG. 10B is a top view of a configuration example of a spectroscopydevice using light in the far-infrared region in the fourth example ofthe present invention.

FIG. 10C is a plan view showing an irradiation region of a sample in thefourth example of the present invention.

FIG. 11A is a side view of a configuration example of the spectroscopydevice using light in the far-infrared region in a fifth example of thepresent invention.

FIG. 11B is an enlarged view of a surface of a cylindrical lens of anillumination optical system in the fifth example of the presentinvention.

FIG. 11C is an enlarged view of a surface of a cylindrical lens of anillumination optical system in the fifth example of the presentinvention.

FIG. 12A is a view for explaining a groove structure of a surface of acylindrical lens of an illumination optical system in the fifth exampleof the present invention.

FIG. 12B is a view for explaining a groove structure of the surface ofthe cylindrical lens of the illumination optical system in the fifthexample of the present invention.

FIG. 12C is a view for explaining a groove structure of the surface ofthe lens used in an optical system for adjusting an optical path lengthand a far-infrared light imaging optical system in the fifth example ofthe present invention.

FIG. 13 is a view showing a configuration example of a far-infraredspectroscopy device using light in the far-infrared region in therelated art.

DESCRIPTION OF EMBODIMENTS

Hereinafter, examples of the present invention will be described withreference to the accompanying drawings. The accompanying drawingsillustrate specific examples consistent with the principles of theinvention, but these examples are for an understanding of the presentinvention and are not used to limit interpretation of the presentinvention in any way.

The following example relates to a far-infrared spectroscopy device thatanalyzes a sample by using light in a far-infrared region in an analysisof the content of a chemical substance component in a sample or aninspection process such as inspection of a heterogeneous component or acontaminant. Here, the light in the far-infrared region is, for example,light ranging from 25 μm to 4 mm in a wavelength. Numerical ranges ofvarious wavelengths exist as the definition of “far-infrared region”,but the light in the far-infrared region described below should beinterpreted as the widest range within the range defined in every field.In addition, the term “terahertz wave” is assumed to be included in thefar-infrared region described above.

FIRST EXAMPLE

FIGS. 1A and 1B show examples of the overall configuration of afar-infrared spectroscopy device according to a first example. As anexample, the far-infrared spectroscopy device is a device that measuresthe absorption spectrum of a sample 200 by using light transmittedthrough sample 200.

The far-infrared spectroscopy device includes a variable wavelengthfar-infrared light source 100 that generates far-infrared light, anillumination optical system 150 that emits far-infrared light to thesample 200, a sample stage 202 that mounts the sample 200, afar-infrared light imaging optical system 170 that images thefar-infrared light from the sample 200 onto a nonlinear optical crystalfor detection 132, the nonlinear optical crystal for detection 132 thatconverts the far-infrared light from sample 200 into near-infrared lightby using pump light, a photo-detector (sensor) 290, a signal processingunit 400, and a control unit 500.

The variable wavelength far-infrared light source 100 includes a lightsource 110 for the pump light 115, a variable wavelength light source120 for a seed light 125, an incident angle adjusting optical system121, and a nonlinear optical crystal (a nonlinear optical crystal forgenerating far-infrared light) 130. As the variable wavelengthfar-infrared light source 100, a configuration that two beams of laserlight (pump light 115 and seed light 125) of different wavelengths areincident on the nonlinear optical crystal 130 to generate far-infraredlight by difference frequency generation or parametric generation isused.

For example, MgO: LiNbO3 may be used as the nonlinear optical crystal130 and a short-pulse Q switch YAG laser may be used as the light source110 of the pump light 115. In this configuration, the light from thevariable wavelength light source 120 is incident as the seed light 125to the nonlinear optical crystal 130 at a slight angle with respect tothe pump light 115 via the incident angle adjusting optical system 121and the mirror 126. As a result, the far-infrared light may be obtainedby parametric generation.

A Si prism 140 may be attached to the nonlinear optical crystal 130.According to this configuration, the generated far-infrared light may betaken out efficiently. If the wavelength of the seed light 125 ischanged in the range of 1066 nm to 1076 nm and the incident angle to thenonlinear optical crystal 130 is adjusted by the incident angleadjusting optical system 121, the frequency of the generatedfar-infrared light may be changed within the range of about 0.5 THz to 3THz.

The far-infrared light thus obtained is emitted to the irradiationregion 205 on the sample 200 by using the illumination optical system150. FIG. 1C is a plan view showing an irradiation region of the sample200.

The illumination optical system 150 is an anamorphic imaging opticalsystem consisting of at least three cylindrical lenses 152, 154, and156. Here, “anamorphic” means that optical characteristics are differentfrom each other in two orthogonal planes including an optical axis.Specifically, in the “imaging optical system”, the term “anamorphic”means that magnifications are different from each other in twoorthogonal planes including the optical axis.

The illumination optical system 150 will be described more specifically.In the plane of FIG. 1B, the cylindrical lenses 152 and 154 have opticalpower. The emission region of the far-infrared light is arranged on thefront focal plane of the cylindrical lens 152. Here, the light-emittingregion is a linear light-emitting region (for example, a linear regionpassing through B0 and A0 in FIG. 2) along the pump light 115. Inaddition, the cylindrical lens 154 is arranged so that the rear focalplane of the cylindrical lens 152 and the front focal plane of thecylindrical lens 154 coincide. Further, the sample 200 is arranged onthe rear focal plane of the cylindrical lens 154.

An aperture diaphragm may be provided on the rear focal plane of thecylindrical lens 152 (that is, also the front focal plane of thecylindrical lens 154). According to this configuration, an opticalsystem that is exactly double-side telecentric within the plane of FIG.1B is obtained, but an aperture diaphragm is not indispensable here. Byusing the aperture diaphragm, the range of use of far-infrared light maybe limited.

The illumination optical system 150 is an afocal optical system and mayfunction as a substantially double-side telecentric optical system fromthe spreading characteristics of the far-infrared beam emitted from thenonlinear optical crystal 130.

In addition, in the plane of FIG. 1A, the cylindrical lens 156 includesoptical power. The cylindrical lens 156 is arranged so that thelight-emitting region of far-infrared light is imaged on the sample 200.

The illumination optical system 150 is an imaging optical system thatcollimates far-infrared light in a first cross section including theoptical axis of the far-infrared light from the variable wavelengthfar-infrared light source 100 and condenses the far-infrared light againon the sample 200 surface and is a condensing optical system thatcondenses the far-infrared light from the variable wavelengthfar-infrared light source 100 onto the sample 200 surface in a secondcross section orthogonal to the first cross section. In particular, thefar-infrared light emitted from the variable wavelength far-infraredlight source 100 is a linear light source (FIG. 2) along the beam of thepump light 115. The far-infrared light emitted from the variablewavelength far-infrared light source 100 spreads in the plane of FIG.1A, but becomes parallel light flux in the plane of FIG. 1B. In theplane of FIG. 1A, the far-infrared light is converged to the irradiationregion on the sample 200 by the cylindrical lens 156. On the other hand,in the plane of FIG. 1B, the far-infrared light is temporarily condensedby the cylindrical lens 152, converted into parallel light flux again bythe cylindrical lens 154, and emitted onto the sample 200. In thismanner, the illumination optical system 150 may image a linearlight-emitting region formed by the variable wavelength far-infraredlight source 100 on the sample 200 by reducing the longitudinaldirection of the linear region.

The illumination optical system 150 is afocal in a first cross sectionincluding the optical axis of the far-infrared light from the variablewavelength far-infrared light source 100. By setting the illuminationoptical system 150 as an afocal system (that is, the cylindrical lens154 is arranged so that the rear focal plane of the cylindrical lens 152coincides with the front focal plane of the cylindrical lens 154),almost parallel far-infrared light in the plane of FIG. 1B to begenerated in the nonlinear optical crystal 130 may be emitted to thesample 200 as parallel light flux as it is. In addition, thefar-infrared light transmitted through the sample 200 may be efficientlycaptured into the far-infrared light imaging optical system 170.

In this manner, by setting the illumination optical system 150 as theimaging optical system, it becomes possible to secure the stability ofthe illumination when the wavelength of the far-infrared light emittedfrom the variable wavelength far-infrared light source 100 is changed.That is, by setting the illumination optical system 150 as the imagingoptical system, the irradiation position of the far-infrared light onthe sample 200 does not depend on the wavelength of the far-infraredlight from the variable wavelength far-infrared light source 100.

In order to change the wavelength of the far-infrared light, theincident angle to the nonlinear optical crystal 130 is adjusted whilechanging the wavelength of the seed light 125. At that time, theemission direction of far-infrared light to be generated changes in thein-plane direction of FIG. 1B (for example, θ1 to θ2 in FIG. 1B). Alsoin this case, by setting the illumination optical system 150 as theimaging optical system and setting the light-emitting region offar-infrared light and the sample 200 surface to a conjugaterelationship (imaging relation), it is possible to prevent a spot of thefar-infrared light from moving even on the sample 200 surface. Even ifthe wavelength of far-infrared light is changed, since the irradiationposition of the sample 200 is not shifted, there is no change in theamount of illumination light, and stable illumination may be secured. Onthe other hand, when the illumination optical system 150 is not used asthe imaging optical system, there is a possibility that the illuminationposition of the illumination light may be totally different, makingstable capturing difficult. However, this is not limited to a case ofcapturing with a fixed wavelength and a case where the change range ofthe wavelength is small and the change of the direction of far-infraredlight is sufficiently small.

In addition, by setting the illumination optical system 150 as ananamorphic optical system, the linear light-emitting region of thevariable wavelength far-infrared light source 100 may be illuminatedwith the sample 200 as a spot by reducing the longitudinal direction ofthe linear region. According to this configuration, two-dimensional datamay be obtained by scanning the spot on the sample 200 as describedbelow.

The sample 200 to be captured is mounted on the stage 202. The stage 202includes a mechanism movable in at least one direction. For example, thestage 202 may move in an x direction in FIG. 1. According to thisconfiguration, by moving the sample 200 in the x direction, theirradiation region 205 may be scanned on the surface of the sample 200,and the data of the linear region of the sample 200 may be acquired.Further, the stage 202 may be an xy stage movable in the x direction anda y direction. Two-dimensional data (image) of a wider region of thesample 200 may also be obtained by combining scanning in the x directionand scanning in the y direction.

The wavelength of the far-infrared light transmitted through the sample200 is converted to near-infrared light in the vicinity of a wavelengthof 1066 nm to 1076 nm by the nonlinear optical crystal for detection132. The converted near-infrared light is photoelectrically converted bythe photo-detector 290 that is sensitive to near-infrared light anddetected as a detection signal. The photo-detector (sensor) 290 fornear-infrared light may be a light-receiving element (1D array detector)in which a plurality of light-receiving elements are arranged in aone-dimensional array, or may be a light-receiving element (2D arraydetector) in which a plurality of light-receiving elements are arrangedin a two-dimensional array. The 1D array detector and the 2D arraydetector for near-infrared light are comparatively easy to obtain, theresponse speed thereof is fast, and may be used at a room temperature.Therefore, these detectors are suitable for industrial applications.

As in the above example, in a case of converting the wavelength offar-infrared light to near-infrared light by using the nonlinear opticalcrystal for detection 132, a part of the pump light 115 is branched andis incident on the nonlinear optical crystal for detection 132 afteradjustment. For example, the pump light 115 is split into transmittedlight and reflected light by a polarization beam splitter (hereinafter,referred to as PBS) 127. The transmitted light transmitted through thePBS 127 is incident on the nonlinear optical crystal 130. The reflectedlight (pump light for wavelength conversion) 235 reflected by the PBS127 is incident to the nonlinear optical crystal for detection 132.

The pump light 235 for wavelength conversion is incident on thenonlinear optical crystal for detection 132 at the same timing as thetiming of the pulse of far-infrared light transmitted through the sample200 being incident. For this reason, illustration is omitted on theoptical path of the pump light 235 for wavelength conversion, but adelay optical system (for example, an optical path length correctionstage and the like) for adjusting the timing of the optical pulses, anda half-wavelength plate for adjusting a polarization direction areprovided as necessary. According to this configuration, it is possibleto use a clean beam having a profile in wavelength conversion uponfar-infrared light detection. As a result, it is possible to increasethe efficiency of wavelength conversion and increase the detectionsensitivity.

The far-infrared light transmitted through the sample 200 is guided tothe nonlinear optical crystal for detection 132 by using thefar-infrared light imaging optical system 170. For example, thefar-infrared light imaging optical system 170 is an afocal imagingoptical system including at least two lenses 177 and 179. The lens 177is arranged on a stage 178 a. In addition, the lens 179 is arranged onthe stage 178 b. The stages 178 a and 178 b are movable in at least onedirection (here, the y direction). A mechanism for moving at least thelens on the sample 200 side out of the two lenses 177 and 179 in atleast one direction may be provided. The sample 200 surface is imagedwithin the nonlinear optical crystal for detection 132 through an Siprism 142. For the nonlinear optical crystal for detection 132, LiNbO3or MgO: LiNbO3 may be used. Each light beam passing through thenonlinear optical crystal for detection 132 and the nonlinear opticalcrystal 130 is received and processed by a termination processing unit240 (see FIG. 1B).

In a case where the output of the light source 110 for the pump light115 has no margin, the pump light passing through the nonlinear opticalcrystal 130 may be guided to the nonlinear optical crystal for detection132 and reused (see, for example, FIG. 6). Since the quality of the beamof the pump light used for wavelength conversion deteriorates, thedetection efficiency decreases, but it is possible to efficiently usepump light for generation of far-infrared light and wavelengthconversion to near infrared.

The signal processing unit 400 captures the signal photoelectricallyconverted by the photo-detector 290. The signal processing unit 400generates a signal proportional to the light transmitted through thesample 200 and the distribution image thereof based on the positioninformation of the stage 202 at the time of signal acquisition. Thesignal processing unit 400 may calculate an absorption spectrum andobtain a two-dimensional distribution (absorption spectrum image) of theabsorption spectrum by comparing the acquired image data with thespectral image data (reference data) when there is no sample stored inthe storage region of the signal processing unit 400.

The control unit 500 controls the entire device. For example, thecontrol unit 500 controls the variable wavelength far-infrared lightsource 100, the stages 202, 178 a, 178 b, and the signal processing unit400. In addition, the control unit 500 functions as a user interface.For example, the control unit 500 may include a display unit thatdisplays the signal and data (spectral information) acquired by thesignal processing unit 400. In a case of acquiring data of the sample200 by fixing the wavelength, the control unit 500 controls the variablewavelength far-infrared light source 100 to generate a specifiedfar-infrared light and controls the synchronization of the movement ofthe stage 202 and the data acquisition by the photo-detector 290. Inaddition, in a case of acquiring data of the sample 200 by changing thewavelength, the control unit 500 sets the wavelength of the variablewavelength far-infrared light source 100 and controls thesynchronization between the movement of the stage 202 and the dataacquisition by the photo-detector 290.

In the present example, the short-pulse Q switch YAG laser is used asthe light source 110 for the pump light of the variable wavelengthfar-infrared light source 100, but the present invention is not limitedthereto. As long as the line width of a fundamental spectrum is narrow,a mode-locked laser may be used as light source 110 for pump light.Since the mode-locked laser has high repetition rate, faster measurementbecomes possible.

Here, how far-infrared light is generated will be described withreference to FIG. 4A. FIG. 4A shows an example in which MgO: LiNbO3 isused as the nonlinear optical crystal 130 and far-infrared light isgenerated by parametric generation.

The pump light 115 is incident on the nonlinear optical crystal 130, andon the contrary, the seed light 125 is incident on the nonlinear opticalcrystal 130 at an angle θ. By setting the wavelength of the seed lightand the angle θ with respect to the pump light so as to satisfy thefollowing conditions, far-infrared light 145 may be generated with highefficiency. The frequency (ω_(T)) of the far-infrared light 145 to begenerated in the nonlinear optical crystal 130 is obtained by thefollowing expression from the law of conservation of energy by using therespective frequencies (ω_(p) and ω_(s)) of the pump light 115 and theseed light 125 (however, ω is an angular frequency).

ω_(T)=ω_(p)−ω_(s)  [Expression 1]

On the other hand, the generation efficiency of the far-infrared light145 increases when the law of conservation of momentum holds. That is,high efficiency is obtained when the following relational expression andthe condition (phase matching condition) in FIG. 4B are establishedbetween the emission direction of the far-infrared light 145 and thedirection of the pump light 115 and the seed light 125. Here, the wavevectors of the far-infrared light 145, pump light 115, and seed light125 are k_(T), k_(p), and k_(s).

k _(p) =k _(s) +k_(T)  [Expression 2]

Accordingly, far-infrared light (terahertz light) may be generated withhigh efficiency by setting the wavelength and the incident direction (θ)of the seed light 125 so as to satisfy these conditions.

In the present example, the incident angle adjusting optical system 121adjusts the incident angle of the seed light 125 to the nonlinearoptical crystal 130. FIG. 5 shows a configuration example of theincident angle adjusting optical system 121.

The incident angle adjusting optical system 121 consists of a lens 122,an optical deflector 123, and an imaging optical element 124. Light fromvariable wavelength light source 120 is guided through a fiber 128.Light from the fiber 128 forms a beam waist near the front focal planeof the imaging optical element 124 via the lens 122 and the opticaldeflector 123. According to this configuration, the beam that passedthrough the imaging optical element 124 becomes a beam with a longRayleigh length (that is, close to a collimated state) and is incidenton the nonlinear optical crystal 130.

On the other hand, the imaging optical element 124 is configured toimage the surface of the optical deflector 123 on the incident plane ofthe nonlinear optical crystal 130. As a result, it is possible torealize a condition that, when the beam is shaken by the opticaldeflector 123, the beam position does not change on the incident surfaceof the nonlinear optical crystal 130 but only the incident anglechanges.

As the optical deflector 123, a reflective deflector such as agalvanometer mirror or a mirror using a MEMS technique may be used, or atransmissive optical deflector may be used. That is, as long as theangle may be controlled, any kind of optical deflector 123 may be used.

In addition, in this example, a concave mirror is used as the imagingoptical element 124. However, since it is sufficient that the incidentsurface of the optical deflector 123 and the incident surface of thenonlinear optical crystal 130 may be in an imaging relationship, a lensmay be used as the imaging optical element 124. If a reflective opticaldeflector such as a galvano mirror is used as the optical deflector 123and a concave mirror is used as the imaging optical element 124, it ispossible to fold and convolve the optical path, thus it is possible toform the incident angle adjusting optical system 121 compactly.

In a case where the incident angle adjusting optical system 121 islinearly mounted, a transmissive optical deflector may be used as theoptical deflector 123 and a lens may be used as the imaging opticalelement 124. Further, depending on the mounting restrictions, one of theoptical deflector 123 and the imaging optical element 124 may consist ofa reflective optical element, and the other may consist of atransmissive optical element.

In this example, a single imaging optical element, such as a single lensor a single concave mirror, is available as the imaging optical element124. Therefore, it is possible to form the optical system compactly.

In addition, according to the incident angle adjusting optical system121 of this example, when the wavelength of variable wavelength lightsource 120 is changed, by setting the incident angle θ to the nonlinearoptical crystal 130 of the seed light 125 by controlling with theoptical deflector 123, it is possible to set the incident angle θ to thenonlinear optical crystal 130 with high accuracy. Therefore, stabilityof the far-infrared light output and stability of the absorptionspectrum measurement may be improved when the wavelength of the seedlight 125 is changed. As a result, highly accurate quantitativemeasurement may be performed.

Next, how the far-infrared light propagates will be described withreference to FIG. 2. The far-infrared light emitted from the variablewavelength far-infrared light source 100 is a linear light source alongthe beam of the pump light 115. Since this linear light source is tiltedwith respect to the optical axis (a z axis in FIG. 2), attention isrequired to guide the beam to the nonlinear optical crystal fordetection 132.

FIG. 2 shows an optical path until the far-infrared light generated inthe nonlinear optical crystal 130 is incident on the nonlinear opticalcrystal for detection 132 via the illumination optical system 150, thesample 200, and the far-infrared light imaging optical system 170.

Here, two points (A₀ and B₀) on a linear light source of far-infraredlight along the beam of the pump light 115 will be described as anexample. The points A₀ and B₀ are imaged in the vicinity of the sample200 by the illumination optical system 150, but since the point B₀ isaway from the illumination optical system 150 than the point A₀, thepoint B which is the image is formed at a position closer to theillumination optical system 150 in the vicinity of the sample 200. Thesepoints A and B are guided again to the nonlinear optical crystal fordetection 132 in the far-infrared light imaging optical system 170.Here, since the point B is away from the far-infrared light imagingoptical system 170 than the point A, the point B′ which is the image isformed at a position closer to the far-infrared light imaging opticalsystem 170 than the point A′ within the nonlinear optical crystal fordetection 132. Therefore, the nonlinear optical crystal 130 and thenonlinear optical crystal for detection 132 are arranged so that theincident direction of the pump light 115 in the nonlinear opticalcrystal 130 and the incident direction of the pump light 235 in thenonlinear optical crystal for detection 132 are substantially parallel.Since the incident direction of the pump light 115 in the nonlinearoptical crystal 130 and the incident direction of the pump light 235 inthe nonlinear optical crystal for detection 132 are substantiallyparallel, both the points A′ and B′ may be superimposed on the beam ofthe pump light 235 for wavelength conversion, and thus far-infraredlight may be efficiently converted to near-infrared light.

Next, a method of adjusting the incident angle when far-infrared lighttransmitted through the sample 200 is incident on the nonlinear opticalcrystal for detection 132 will be described with reference to FIG. 3.

In order to convert far-infrared light transmitted through the sample200 to near-infrared light in the nonlinear optical crystal fordetection 132 with high efficiency, optimization of the incident angleof the far-infrared light transmitted through sample 200 to thenonlinear optical crystal for detection 132 is necessary. In the presentexample, adjustment of the incident angle is performed by adjusting thepositions of the lenses 177 and 179 constituting the far-infrared lightimaging optical system 170.

FIGS. 3A and 3B show the far-infrared light imaging optical system 170.The points A and B are two points on the sample 200, and these points Aand B are imaged at points A′ and B′ within the nonlinear opticalcrystal for detection 132 by the far-infrared light imaging opticalsystem 170.

FIG. 3A shows a reference state, and FIG. 3B shows a state in which theincident angle to the nonlinear optical crystal for detection 132 of thefar-infrared light transmitted through the sample 200 is changed withrespect to the state in FIG. 3A. First, by driving the stage 178 a, thelens 177 is moved by a distance S in a −y direction. In this state, in acase where the lens 179 is not moved, the image points A′ and B′ of thepoints A and B move in the −y direction within the nonlinear opticalcrystal for detection 132. Therefore, in order to correct this shift, bydriving the stage 178 b, the lens 179 is moved by a distance S′ in a +ydirection. By performing such control, it is possible to change (adjust)the incident angle θ of the far-infrared light incident on the points A′and B′, with the positions of the points A′ and B′ in the same state asin FIG. 3A. The distances S and S′ are not necessarily the same valueand may not be the same value due to the relationship of a focal length.

The effect of this example will be described below. In the related art,changing the frequency of a light source for generating far-infraredlight changes the direction in which the far-infrared light comes out,an irradiation position to a sample changes, and there is a problem thatvariations in component distribution and change in signal detectionefficiency deteriorate the accuracy of quantitative analysis. On theother hand, according to this example, illumination light from a lightsource is emitted onto a target to be analyzed (sample 200) with ananamorphic imaging optical system, the wavelength of the transmittedlight (or reflected light) from the sample 200 is converted by using thenonlinear optical crystal for detection 132 and detected by using thephoto-detector 290. According to this configuration, it is possible toreduce a shift of the irradiation position of far-infrared light due tofrequency change. Therefore, variations in component distribution andchange in signal detection efficiency do not occur, and the accuracy ofquantitative analysis may be improved.

In addition, if the optical system is made complicated to reduce theshift of the irradiation position of the sample, the attenuation of thefar-infrared light by the optical system is remarkable, and it isdifficult to measure via the shield. On the other hand, in this example,the configuration for reducing the shift of the irradiation position ofthe sample is a simple configuration, the attenuation of thefar-infrared light is also small, and thus the analysis through theshield is also possible. According to this example, it is possible torealize quantitative analysis of samples in various forms includingpowder irrespective of the presence or absence of a shield.

SECOND EXAMPLE

FIG. 6 shows a configuration of the spectroscopy device in a secondexample. Constituent elements described in the above example are denotedby the same reference numerals, and description thereof will be omitted.The difference from the first example in FIG. 1 is mainly (i) theconfiguration of the illumination optical system 150 and thefar-infrared light imaging optical system 170, and (ii) arrangement ofthe nonlinear optical crystal for detection 132, and (iii) that the pumplight used for generating far-infrared light passing through thenonlinear optical crystal 130 is guided to the nonlinear optical crystalfor detection 132 and reused.

The light source 110 of the pump light 115 includes, as a maincomponent, a short-pulse Q switch YAG laser 111, a polarizationsplitting system consisting of a polarization beam splitter(hereinafter, referred to as PBS) 114 and a quarter wavelength plate116, and an amplifier unit (here, solid-state amplifier 118 in thiscase) that amplifies the output of the laser. For example, the outputbeam of the YAG laser 111 is collimated by the lens 112 and is amplifiedby the solid-state amplifier 118 via the polarization separation systemconstituted by the PBS 114 and the quarter wavelength plate 116.

More specifically, the output beam of the YAG laser 111 is reflected bythe mirror 181, collimated by the lens 112, reflected by the mirror 182,and incident on the PBS 114. The beam that passed through the PBS 114 isreflected by the mirror 183 via the quarter wavelength plate 116 and thesolid-state amplifier 118. The reflected beam is incident on the PBS 114via the solid-state amplifier 118 and the quarter wavelength plate 116.Thereafter, the beam is emitted as the pump light 115 from the PBS 114via the mirror 184. By using the solid-state amplifier 118, the outputof the YAG laser 111 is amplified, and powerful far-infrared light of kWlevel may be taken out from the nonlinear optical crystal 130 with peakpower.

In this example, the illumination optical system 150 is constituted by acylindrical lens 151 and a condensing lens 155. The far-infrared lightemitted from the variable wavelength far-infrared light source 100having a linear light-emitting region along the beam of the pump light115 becomes parallel light flux by the cylindrical lens 151 and iscondensed on the spot on the sample 200 by the condensing lens 155.

The far-infrared light transmitted through the sample 200 is guided tothe nonlinear optical crystal for detection 132 by the far-infraredlight imaging optical system 170. The far-infrared light imaging opticalsystem 170 is an imaging optical system that images the sample 200surface in the nonlinear optical crystal for detection 132. Thefar-infrared light imaging optical system 170 is constituted by a lens176, a mirror 172, and a condensing lens 174. Specifically, thefar-infrared light transmitted through the sample 200 is collimated bythe lens 176, reflected by the mirror 172, and condensed on thenonlinear optical crystal for detection 132 by the condensing lens 174.

FIG. 7A shows spots 205 a, 205 b, and 205 c of far-infrared light on thesample 200 surface. As shown above, the far-infrared light emitted fromthe variable wavelength far-infrared light source 100 changes theemission direction thereof within the plane of FIG. 6 when wavelengthscanning is performed (for example, θ1 to θ2 in FIG. 6). Therefore, forexample, in a case where the wavelength of the seed light 125 is changedand the frequency of the far-infrared light to be generated changes from1 THz to 3 THz, on the sample 200 surface, the spot of far-infraredlight becomes a position of 205 c at 1 THz on the low frequency side, aposition of 205 b at 2 THz at the intermediate frequency, a position of205 a at 3 THz on the high frequency side. In a case where the frequencyis changed in this manner, the irradiation position on the sample 200 isdifferent. Therefore, the position dependence of the concentrationdistribution affects the absorption spectrum, and an accurate spectrummay not be obtained. Therefore, in this example, the stage 202 of sample200 is controlled in accordance with the frequency of far-infrared lightso that the irradiation position does not change even if the frequencyof far-infrared light changes.

When the wavelength of the seed light 125 is changed, the control unit500 moves the stage 202 in accordance with the change in the irradiationposition of the far-infrared light that may occur on the sample 200surface. With this configuration, when the wavelength of thefar-infrared light from the variable wavelength far-infrared lightsource 100 changes, far-infrared light may be emitted to the sameposition of the sample 200. That is, the irradiation position offar-infrared light on the sample 200 does not depend on the wavelengthof far-infrared light from the variable wavelength far-infrared lightsource 100.

For example, in a case where the wavelength of the far-infrared lightfrom the variable wavelength far-infrared light source 100 is 1 THz onthe low-frequency side, since the spot of the far-infrared light becomesa position of 205 c, the control unit 500 drives the stage 202 in the −ydirection (see FIG. 7C). In addition, in a case where the wavelength ofthe far-infrared light from the variable wavelength far-infrared lightsource 100 is 3 THz on the high-frequency side, since the spot of thefar-infrared light becomes a position of 205 a, the control unit 500drives the stage 202 in the +y direction (see FIG. 7B). When thewavelength of the seed light 125 is changed, the control unit 500 isconfigured to move the stage 202 in accordance with the change in theirradiation position of the far-infrared light that may occur on thesample 200 surface so as to emit the spot of far-infrared light at thesame position of the sample (the center in FIGS. 7A to 7C). According tothis configuration, it is possible to avoid the influence of theposition dependence of the concentration distribution on the absorptionspectrum, thereby enabling accurate and stable spectrum measurement.

In addition, in this example, the pump light transmitted through thenonlinear optical crystal 130 is used as pump light for wavelengthconversion. Specifically, the pump light transmitted through thenonlinear optical crystal 130 is reflected by the mirror 210 and guidedto the nonlinear optical crystal for detection 132 as the pump light 235for wavelength conversion. This configuration makes it possible toefficiently use the pump light 115 and is suitable when the power of thepump light 115 has no margin. However, for the purpose of reuse, thepump light 235 for wavelength conversion may be deformed in a timewaveform. For that reason, conversion efficiency to near-infrared lightand beam quality of converted near-infrared light may decrease. In acase where the power of the pump light 115 has a margin, it is desirableto branch the pump light into two for generating far-infrared light andconverting to near-infrared light as shown in FIG. 1.

FIG. 8 shows another configuration example of the light source 110 forthe pump light 115 used in the variable wavelength far-infrared lightsource 100. The configuration of the light source 110 shown in FIG. 8 isalso applicable to other examples. In FIG. 8, the same constituentelements as those in FIG. 6 are denoted by the same reference numerals.

The variable wavelength far-infrared light source 100 in this exampleincludes lenses 119 a and 119 b for correcting a thermal lens effectoccurring in the solid-state amplifier 118. That is, the difference fromthe example shown in FIG. 6 is that the two lenses 119 a and 119 b forcorrecting the thermal lens effect are arranged between the solid-stateamplifier 118 and the mirror 183. In a case where the output of the YAGlaser 111 is amplified by using the solid-state amplifier 118, theamplified beam may be squeezed by the thermal lens effect and damage thePBS 114 and the quarter wavelength plate 116 in some cases. Therefore,an optical system that corrects the thermal lens to be generated in thesolid-state amplifier 118 is provided between the solid-state amplifier118 and the folded mirror 183. Amplified light emitted from thesolid-state amplifier 118 is incident on the lenses 119 a and 119 b forcorrection, and light transmitted through the lenses 119 a and 119 b isincident on the PBS 114 and the quarter wavelength plate 116.

Specifically, a combination of the concave lens 119 a and the convexlens 119 b is arranged between the solid-state amplifier 118 and themirror 183 so that the composite focal length becomes negative. Further,the synthesized principal plane is configured to be located near thesolid-state amplifier 118. According to this configuration, it ispossible to return the convergent beam by the thermal lens to a parallelbeam and hit the mirror 183, and thus it is possible to realize aconfiguration in which the beam reflected by the mirror 183 follows theopposite optical path, is incident on the solid-state amplifier 118again, and is returned back to the parallel light flux by the thermallens and emitted. Since the beam output from the solid-state amplifier118 is parallel light flux, energy density on the polarization beamsplitter 114 and the quarter wavelength plate 116 may be suppressed, andthus damage may be avoided.

Further, by configuring the optical system that corrects the thermallens effect by the two lenses 119 a and 119 b, it is possible to adjustthe focal length of the correction system. Even in a case where thethermal lens effect is changed by changing the gain of the solid-stateamplifier 118, it is possible to deal with by adjusting the surfacespacing of the lenses 119 a and 119 b and to constitute a system with ahigh degree of freedom. In addition, in this example, an optical systemthat corrects the thermal lens is provided between the solid-stateamplifier 118 and the folding mirror 183, which may be expected to havean effect of avoiding damage to the optical system. At this position,the beam passes through the solid-state amplifier 118 only once and thebeam power has not increased so much yet.

In the related art, since the frequency variable coherent light sourcerequires a high output laser, the optical system is damaged by thethermal lens effect, and thus it is difficult to perform stablemeasurement. On the other hand, in the present example, the two lenses119 a and 119 b are provided as an optical system that corrects thethermal lens effect, thereby avoiding damage to the optical system andenabling stable measurement. The configuration of the light source 110for the pump light 115 shown in FIG. 8 may also be applied to otherexamples.

THIRD EXAMPLE

FIGS. 9A and 9B show examples of the overall configuration of thefar-infrared spectroscopy device according to a third example. FIG. 9Cis a plan view showing an irradiation region of a sample in the thirdexample. Constituent elements described in the above example are denotedby the same reference numerals, and description thereof will be omitted.

In the far-infrared spectroscopy device of this example, an absorptionspectrum is measured by using the reflected light of the sample 200.With the sample 200 surface as the center, the constituent elements fromthe variable wavelength far-infrared light source 100 to theillumination optical system 150 and the constituent elements after thefar-infrared light imaging optical system 170 are inclined with respectto the plane of the sample 200. In the far-infrared spectroscopy deviceof this example, far-infrared light is incident obliquely on the sample200, and the reflected light from the sample 200 is detected.

According to this configuration, by arranging the illumination opticalsystem 150 and the far-infrared light imaging optical system 170inclined with respect to the surface of the sample 200, reflected lightfrom the sample 200 may be detected. As a result, it is possible tomeasure the sample 200 having a low transmittance or to measure thespectral characteristics of the surface of the sample 200. A mechanismcapable of changing the incident angle from the illumination opticalsystem 150 and the angle to the far-infrared light imaging opticalsystem 170 may be provided. As a result, it is also possible to measureincident angle dependence of the spectral characteristics.

FOURTH EXAMPLE

FIGS. 10A and 10B show examples of the overall configuration of afar-infrared spectroscopy device according to a fourth example. FIG. 10Cis a plan view showing an irradiation region of a sample in the fourthexample. Constituent elements described in the above example are denotedby the same reference numerals, and description thereof will be omitted.

In the present example, each of the illumination optical system 150 andthe far-infrared light imaging optical system 170 further includes anoptical system that bends the optical path of far-infrared light andcorrects the optical path length of far-infrared light. Morespecifically, between the illumination optical system 150 and a sample200 b, an optical system 158 a that adjusts the optical path length anda mirror 159 a are arranged. Further, a mirror 159 b and an opticalsystem 158 b that adjusts the optical path length are arranged betweenthe sample 200 b and the far-infrared light imaging optical system 170.In addition, in this example, the sample 200 b is arranged on an yzplane of a stage 202 b. According to this configuration, the absorptionspectrum measurement using the reflected light of the sample 200 bbecomes possible.

If the optical path of the far-infrared light is simply bent by themirrors 159 a and 159 b and the reflected light of the sample isdetected, the optical path length of the far-infrared light iselongated, and thus a focal point does not match on the sample surface.Hence, as a feature of the present example, the optical system 158 a forcorrecting the optical path length is arranged in the front stage of themirror 159 a, and the optical system 158 b for correcting the opticalpath length is arranged in the subsequent stage of the mirror 159 b.Since the optical system is arranged symmetrically with respect to thesample 200 b, the optical systems 158 a and 158 b may be the same. Sincethe optical systems 158 a and 158 b need to extend the optical pathlength, it is possible to constitute, for example, with one concavelens. According to this configuration, since additional parts aresuppressed to the minimum, it is effective when the mounting constraintis severe.

On the other hand, the optical systems 158 a and 158 b may beconstituted by a combination of a convex lens and a concave lens.According to this configuration, it is possible to adjust the focallength of the concave lens obtained by combining these lenses byadjusting the spacing between these lenses. Therefore, it is possible touse ready-made lenses. In addition, by adjusting the spacing between thelenses, the focal position may be adjusted.

The difference from the configuration of FIGS. 1A and 1B is only theaddition of the mirrors 159 a and 159 b, and the optical systems 158 aand 158 b that adjust the optical path length, and holding of the sample200 b in the yz plane. Therefore, a mechanism for inserting and removingthe mirrors 159 a and 159 b, and the optical systems 158 a and 158 b anda mechanism for replacing the sample 200 b and the stage 202 b with thesample 200 a and the stage 202 a may be provided (see FIG. 10B). Byremoving the mirrors 159 a and 159 b, and the optical systems 158 a and158 b from the optical path of the far-infrared light and replacing thestage 202 b with the stage 202 a, it becomes possible to easily switchthe measurement by the reflected light to the measurement by thetransmitted light.

FIFTH EXAMPLE

FIG. 11A shows an example of the overall configuration of a far-infraredspectroscopy device according to a fifth example. Constituent elementsdescribed in the above example are denoted by the same referencenumerals, and description thereof will be omitted.

The difference from the example described above is an optical element(for example, a lens) used in the illumination optical system 150 of thefar-infrared light, an optical element (for example, a lens) used in theoptical systems 158 a and 158 b for adjusting the optical path lengths,and that an optical element (for example, a lens) used in thefar-infrared light imaging optical system 170 has a structure forpreventing reflection as shown in FIGS. 12A to 12C. At least one opticalelement used in the illumination optical system 150 and the far-infraredlight imaging optical system 170 includes groove-shaped processing toreduce reflection of far-infrared light.

In FIG. 11A, the far-infrared light to be generated from the nonlinearoptical crystal 130 for far infrared generation mainly consists ofpolarized components that vibrate in a vertical direction (x axisdirection in the drawing) in the plane of the paper. In order to preventreflection, the surface of the above-mentioned optical element isprovided with a plurality of V-shaped grooves whose longitudinaldirection is a direction (y-axis direction) orthogonal to thepolarization direction. The plurality of grooves are formedsubstantially in parallel. FIGS. 11B and 11C are enlarged views of thesurface 152 a of the cylindrical lens 152 of the illumination opticalsystem 150 as an example. The plurality of grooves have a depth d andare formed with a period p.

In the above configuration, it is known that if the period p of thegroove is made sufficiently small with respect to the wavelength offar-infrared light, the period of the groove for incident far-infraredlight is equivalent to a case in which the refractive index of aninterface gradually changes. Therefore, as an example, it is preferablethat the surface of the optical element described above has a groovehaving a period equal to or less than ⅓ of the wavelength offar-infrared light (that is, the spacing of the plurality of grooveshapes is equal to or less than ⅓ of the wavelength of firstfar-infrared light from the variable wavelength far-infrared lightsource 100 or second far-infrared light from the sample 200). Morepreferably, the surface of the optical element described above isprovided with a groove having a period equal to or less than ⅕ of thewavelength of far-infrared light. More preferably, the surface of theoptical element described above has a groove having a period of about1/100 of the wavelength of far-infrared light. For example, sincefar-infrared light of 1 to 3 THz has a wavelength of 100 to 300 um, itis advisable to form a groove having a period of about 10 um which is1/10 of the wavelength. For the purpose of reflection reduction, it isdesirable that the period of the groove is small, but if the period ofthe groove is less than about 10 um, which is 1/10 of the wavelength, itbecomes difficult to ensure the processing accuracy, or the processingcost is high, and thus the practicality deteriorates. Therefore, it isdesirable to set the period of the groove to about 1/10 of thewavelength. It is desirable that a depth d of the groove is about thesame as a period p to about 10 times the period p. This is because, forthe purpose of reflection reduction, it is desirable that the change ina refractive index on the optical element surface is gentle on thewavelength scale. That is, if the groove is shallow, the change in therefractive index is not alleviated and the effect of reducing surfacereflection may not be obtained. On the other hand, if the depth exceeds10 times the period p, processing becomes difficult and shape accuracymay not be maintained. When disturbance occurs in the groove shape,scattered light is generated and the reflection reducing effect isdeteriorated.

Since the reflection on the surface of the optical element increases asthe difference in the refractive index on the surface increases, it ispossible to reduce the surface reflection by making a structureequivalent to a case in which the refractive index gradually changes.When forming a V-shaped groove, a portion keeping an original surfaceshape between adjacent grooves may be formed. As shown in FIG. 11C,between the plurality of groove shapes, a flat plane (w portion) isformed in a direction parallel to a plane orthogonal to the optical axisof the first far-infrared light from the variable wavelengthfar-infrared light source 100 or the second far-infrared light from thesample 200. The width of the flat plane is larger than the wavelength ofthe visible light. According to this configuration, the effect ofreducing the surface reflection is weakened, but alignment of theoptical system may be performed by using visible light, and thusadjustment of the optical system becomes easy. If the V-shaped groovesare made to be completely connected (w portion of FIG. 11C does notremain), since the beam of visible light for alignment is refracted bythe groove structure, the visible light does not follow the same opticalpath as far-infrared light. However, if the width of the w portion ismade larger than the wavelength of the visible light for alignment (forexample, 1 um or more), the beam of visible light for alignment passingthrough the w portion follows the same optical path as the far-infraredlight. Therefore, it is possible to perform alignment using a visiblelight beam while reducing surface reflection of far-infrared light.

FIG. 12A shows an example in which the groove structure described aboveis formed on the surfaces of the cylindrical lenses 152 and 154 of theillumination optical system 150. In addition, FIG. 12B shows an examplein which the groove structure described above is formed on the surfacesof the cylindrical lenses 156 of the illumination optical system 150.FIG. 12C shows an example in which the groove structure described aboveis formed on the surfaces of the lenses 177 and 179 used for the opticalsystems 158 a and 158 b that adjust the optical path length, and thefar-infrared light imaging optical system 170. In the example of FIG.12A, grooves orthogonal to the generatrixes of the cylindrical lenses152 and 154 are formed. In addition, in the example of FIG. 12B, agroove parallel to the generatrix of the cylindrical lens 156 is formed.In the example of FIG. 12C, the spherical lens includes a groovestructure. These lenses for far-infrared light are often made of resin.Since resin lenses may be manufactured by molding using a mold, massproduction may be carried out comparatively easily even if there is agroove structure on the surface like this.

The present invention is not limited to the above example, but includesvarious modifications. The above examples have been described in detailin order to describe the present invention in an easy-to-understandmanner and are not necessarily limited to those having all theconfigurations described. In addition, it is also possible to replacepart of the configuration of one example with the configuration ofanother example. In addition, it is also possible to add other exampleconfigurations to an example configuration. In addition, it is alsopossible to add, delete, or replace other configurations for part of theconfiguration of each example.

The processing of the signal processing unit 400 and the control unit500 described above may also be realized by software program codes thatrealize these functions. In this case, a storage medium storing theprogram codes is provided to the system or device, and a computer (orCPU or MPU) of the system or device reads the program codes stored inthe storage medium. In this case, the program codes themselves read fromthe storage medium realize the functions of the example described above,and thus the program codes themselves and the storage medium storing theprogram codes constitute the present invention. As the storage mediumfor supplying such program codes, for example, a flexible disk, aCD-ROM, a DVD-ROM, a hard disk, an optical disk, a magneto-optical disk,a CD-R, a magnetic tape, a nonvolatile memory card, a ROM, and the likeare used.

The processes and techniques described herein are not inherently relatedto any particular device and may be implemented by any suitablecombination of components. Further, various types of devices for ageneral purpose may be used. In some cases, it may be beneficial toconstruct a dedicated device to perform the processing described here.That is, apart of the signal processing unit 400 and the control unit500 described above may be realized by hardware using an electroniccomponent such as an integrated circuit, for example.

Further, in the above example, the control lines and the informationlines indicate what is considered to be necessary for description, andall control lines and information lines are not necessarily shown on aproduct. All the configurations may be mutually connected.

REFERENCE SIGNS LIST

-   100 variable wavelength far-infrared light source-   110 light source-   111 Q switch YAG laser-   112 lens-   114 polarization beam splitter-   115 pump light-   116 quarter wavelength plate-   118 solid-state amplifier (amplifier unit)-   119 a concave lens (thermal lens correction lens)-   119 b convex lens (thermal lens correction lens)-   120 variable wavelength light source-   121 incident angle adjusting optical system-   122 lens-   123 optical deflector-   124 imaging optical element-   125 seed light-   126 mirror-   130 nonlinear optical crystal-   132 nonlinear optical crystal for detection-   140, 142 Si prism-   150 illumination optical system-   170 far-infrared light imaging optical system-   178 a, 178 b stage-   200, 200 a, 200 b sample-   202, 202 a, 202 b stage

1. A far-infrared spectroscopy device comprising: a variable wavelengthfar-infrared light source that generates first far-infrared light; anillumination optical system that irradiates a sample with the firstfar-infrared light; a nonlinear optical crystal for detection thatconverts second far-infrared light from the sample into near-infraredlight by using pump light; and a far-infrared light imaging opticalsystem that images the sample on the nonlinear optical crystal fordetection, wherein the variable wavelength far-infrared light sourceincludes two beams of laser light of different wavelengths and anonlinear optical crystal for generation of far-infrared light, thevariable wavelength far-infrared light source generating far-infraredlight by difference frequency generation or parametric generation, byirradiating the two beams of laser light onto the nonlinear opticalcrystal for generation of far-infrared light, wherein the variablewavelength far-infrared light source includes an incident angleadjusting optical system consisting of an optical deflector and a singleimaging optical element, one of the two beams of laser light is outputlight of a variable wavelength laser, the output light of the variablewavelength laser is configured to forma beam waist at a front focalplane of the single imaging optical element, and the incident angleadjusting optical system adjusts an incident angle of the output lightof the variable wavelength laser to the nonlinear optical crystal forgenerating far-infrared light.
 2. The far-infrared spectroscopy deviceaccording to claim 1, wherein the incident angle adjusting opticalsystem includes a fiber, a lens, an optical deflector, and an imagingoptical element arranged in this order from a side of the variablewavelength laser on an optical path between the variable wavelengthlaser and the nonlinear optical crystal for generation of far-infraredlight, and the lens, the optical deflector, and the imaging opticalelement include one lens, one optical deflector, and one imaging opticalelement, respectively.